In a piston pump of the aforedescribed type, e.g. for the displacement of cryogenic fluids such as deeply cooled liquefied gas and for other purposes, it is known to provide a reciprocating piston which is axially shifted in a cylinder forming a pumping chamber, to draw fluid into this chamber through an intake opening on the intake stroke, and displace this fluid past a pressure valve through the outlet during the discharge stroke.
A pump of this type is described in Linde-Berichte aus Technik und Wissenschaft, 36/1975, pages 15 to 22.
The intake port is axially aligned with the piston and is provided with a valve member in the form of a plate which seats at this port under the force of a spring so that the intake port is blocked during the discharge stroke of the piston and the intake stroke of the piston must initially overcome the spring force and induce the intake valve to open before fluid will be drawn into the pumping chamber.
The function of the spring is to ensure rapid closure of the intake valve so that reversal of the direction of piston movement from the intake to the discharge stroke will not permit discharge of the fluid from the chamber through the intake port. Backflow of fluid from the pumping chamber through the input port and a corresponding reduction in the pumping efficiency is avoided in this manner.
However, since fluid pressure upon the valve member must overcome the spring force during the intake stroke of the piston, opening pressure losses are encountered and intake of fluid is delayed.
It has been sought to overcome this disadvantage by increase in the head of the fluid upstream of the intake port so that the force of the fluid outside the pumping chamber acts upon the valve member in a direction opposite the spring force and thereby contributes to a more rapid opening of the intake valve.
However, increasing the fluid head is difficult in many cases and frequently can only be effected at high cost, if at all, because the practical method of obtaining an increased fluid head on the intake side is to raise the level of the liquid delivered to the intake port.
Frequently it is not possible to elevate the reservoir or vessel containing the liquid to be pumped because of space considerations and in many instances it is neither possible nor practical to provide the vessel at such heights that even when it is close to empty a sufficient force is provided to balance the spring force, or to lower the pumping chamber to the point that the desired head is provided.